Heathrow Harry, the airline doesn't have to take you to court.
If it's a on-off they will probably ignore it.
If they detect persistent abuse of their terms and conditions, which the passenger has explicitly agreed to, they will likely contact the passenger and ask them to cease and desist. If the passenger continues then they will likely blacklist the passenger (Their right to do so is also in their terms and conditions).
If an agent is issuing the tickets they will charge back the differences to the agency. They can and have done this. Few agents are willing to take the risk.
The airline's simply have too much to lose than to allow this to become common practice.
Oh, and good luck keeping any FF miles you made on these trips.
Edited to add - passengers have taken airlines to court attempting to have such airline practice declared illegal. While they have succeeded in low courts they have never succeeded in appeal.